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2016-05-18 20:54  
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A variable Wien bridge provides frequency tuning from 20 Hz to 20kHz. Gain control comes from the positive temperature coefficient of the lamp. When power is applied, the lamp is at a low resistance value, the gain is high, and oscillation amplitude builds. The lamp`s gain-regulating behavior is flat within 0.25 dB over the 20Hz-20kHz range of the circuit. Distortion is below 0.003%. At low frequencies, the thermal time constant of the small normal-mode lamp begins to introduce distortion levels about 0.01 %.

This is because of hunting when the oscillator`s frequency approaches the lamp`s thermal time constant. This effect can be eliminated, at the expense of reduced output amplitude and longer amplitude settling time, by switching to the low-frequency, low-distortion mode. The four large lamps give a longer thermal time constant, and distortion is reduced.

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